1. Field of the Invention
The present invention relates generally to a transmission power control method, a transmitting and receiving apparatus, a base station and a mobile station. More particularly, the invention relates a transmission power control method for controlling a transmission power in a base station and a mobile station in a radio communication system.
2. Description of the Related Art
In a code division multiplexing type mobile communication system, the same frequency is used by a plurality of channels. Therefore, a signal power of a certain channel becomes an interference power to jam other channels. Accordingly, in an up-link channel, in which signal is transmitted from a mobile station and received by the base station, if a signal power to be transmitted from the movable station becomes unnecessarily large, the interference power is increased to cause reduction of a channel capacity.
In order to prevent this, a closed loop type transmission power control is employed. In the closed loop type transmission power control in the up-link channel, the base station measures a reception quality value, such as a signal to interference power ratio, to transmit a control instruction for reducing a transmission power to the mobile station when the reception quality is excessively larger than a control reference value, and to transmit the control instruction for increasing the transmission power to the mobile station when the reception quality value is smaller than the control reference value. Then, the mobile station increases and decreases the transmission power in accordance with the control instruction.
Such transmission power control method has been discussed in detail in U.S. Pat. No. 5,056,109 to Gilhousen et al. for “Method and Apparatus for Controlling Transmission Power in a CDMA Cellular Mobile Telephone System”.
On the other hand, in a down-link channel, the closed loop type transmission power control so that the reception quality value becomes the predetermined control reference value is performed to realize high channel capacity. In the closed loop type transmission power control in the down-link channel, the mobile station measures the reception quality value of the down-link channel to compare with a control reference value to transmit a control instruction for decreasing the transmission power to the base station when the reception quality value is smaller than the reference control value and to transmit the control instruction for increasing the transmission power to the base station when the reception quality value is larger than the reference control value. Then, the base station increases or decreases the transmission power according to the control instruction.
An information bit series to be transmitted and received between the base station and the mobile station is transmitted as divided per time slot (hereinafter simply referred to as slot) both in the up-link channel and in the down-link channel. Then, in the closed loop type transmission power control, the base station and the mobile station transmit the control instructions per slot and the counterpart station updates the transmission power at every occasion of reception of the control instructions. Assuming that the slot is a period about 1 msec, the control instructions are transmitted about thousands times per 1 second to control the transmission power at high speed to compensate fading fluctuation and maintain the reception quality value substantially constant.
However, when the mobile station moves at high speed, it becomes difficult to maintain reception quality constant even with the closed loop control due to high speed fluctuation of fading. On the other hand, it is also possible that the counterpart station erroneously receives the control instruction. Even in such case, reception quality cannot be maintained constant. Accordingly, in order to obtain desired bit error ratio even if the reception quality fluctuates in certain extent, it becomes necessary to set the control reference value in the closed loop type transmission power control at large value. As a result, the transmission power becomes large to decrease channel capacity.
Therefore, by providing error correction coding for the information bit series to perform transmission and reception between the base station and the mobile station, correcting the bit received in error can be realized even when reception error is caused in a part of bits due to instantaneous lowering drop of the reception quality value. Furthermore, a frame is consisted of a plurality of slots and interleaving is provided for exchanging order of the bit series to be transmitted back and forth in an interleaved block with taking one or more frames as the interleaved block.
By this, even if the reception quality value is lowered for a relatively long period to continuously receive bits in error, when the bits can be accurately received in a rate greater than a predetermined rate within the same interleaving block, substantially all bits can be received accurately by using with error correction coding. Thus, bit error ratio can be reduced and hence the reception error ratio of information block, namely block error ratio can be reduced.
Accordingly, for significant fading fluctuation, the block error ratio can be reduced even if the reception quality value cannot be maintained completely constant by the closed loop type transmission power control. Therefore, even if the control reference value is set at relatively small value, the transmission power can be reduced in certain extent with realizing the desired block error ratio.
However, in the conventional closed loop type transmission power control, when the error correction coding and interleaving are used together, the control reference value cannot be set at sufficiently small value to encounter a problem in that the transmission power cannot be reduced satisfactorily. The reason will be discussed as follow.
When the reception quality value is fluctuated due to fading fluctuation, if a ratio of a period, in which a reception quality value is lowered within each interleaving block, is less than or equal to the predetermined ratio, the block error ratio can be efficiently lowered by the error correction coding. However, in the conventional closed loop type transmission power control, the transmission power of the counterpart station is controlled by comparing the reception quality value per slot with the control reference value.
Therefore, in the already received slots in the same interleaving block, even if the slots having low reception quality value is large in number, the transmission power of the counterpart station can be reduced if the reception quality value of the finally received slot is greater than the control reference value. As a result, in the interleaving block, the ratio of the period where the reception quality value decreases becomes large to make probability larger to make it impossible to obtain the bit error ratio lowering effect due to error correction coding.
When the transmission power control, in which the reception quality value is controlled toward the predetermined control reference value per slot, is performed, fluctuation can be caused in a ratio of period to lower the reception quality value in each interleaving block. Therefore, setting the control reference value at relatively large value becomes necessary. Accordingly, even if both of the error correction coding and interleaving are used together, the reference control value becomes sufficiently small to make it impossible to make the transmission power sufficiently small.